The present invention relates generally to the processing of lithographic plates, and more particularly to equipment for punching and bending lithographic plates after they have been through an exposure operation.
This invention has particular (albeit not exclusive) application to lithographic plates used in high-speed press operations to print multi-color images. In such operations, it is essential that the lithographic plates printing different colors be mounted in precise position so that the color images superposed on the printed surface (e.g., newspaper stock) are in exact registration to provide the desired clarity and sharpness. Otherwise, the final image will have ghost-like characteristics.
As will be understood by those skilled in this field, a lithographic plate is formed with flanges and openings which enable it to be mounted on a press. The configuration of these flanges and openings is critical to whether the aforementioned exact registration will be achieved. In conventional lithographic plate punching and bending equipment, a plate is punched and bent so that the image on the plate is generally "square" with the plate when the plate is mounted on the press. However, this does not necessarily result in the desired exactness of registration. For example, if the press on which the plate is mounted is out of alignment with another press in the line, or if the plate-mounting cylinder of the press is slightly deformed, the image printed by the plate on that press will be slightly out of registration with respect to other images printed by plates on other presses in the line. Also, in high volume applications where several identical plates are mounted side-by-side on each press, the multiple side-by-side images formed by the plates may tend to "fan out" as the web of printed material progresses from one press to the next. As a result, the images printed by the plates on the next press may not be in precise registration with the images previously printed on the substrate.
Conventional punching and bending equipment does not take into account any misalignment or deformity associated with the particular press on which a plate is to be mounted. Nor does this equipment compensate for "fan out", or for other factors which can produce images which are out of registration with respect to one another. There is a need, therefore, for equipment which can "match" a plate to the particular press on which it will be mounted so that the image printed by the plate is positioned to achieve the desired preciseness of image registration.
Another deficiency of conventional plate punching and bending equipment is that such equipment is not readily adjustable to vary the angle at which the plate mounting flanges are bent (this angle may vary between presses). Similarly, it is also time-consuming to adjust the "bend-to-bend" dimension of a plate (i.e., the distance from the head end of the plate to the tail end of the plate). There is a need, therefore, for such equipment which is readily adjustable in these respects, yet highly accurate.